“A lot of people interpreted the first study as causal because it’s congenial to their goals,” said Chelsea L. Shover, a postdoctoral fellow in psychiatry who was part of the Stanford research team. “It did not say that one is causing the other.”

The National Institute on Drug Abuse analyzed data from the National Epidemiologic Survey on Alcohol and Related Conditions and found respondents who reported past-year marijuana use in their initial interview had 2.2 times higher odds than nonusers for having a prescription opioid use disorder and 2.6 times greater odds of abusing prescription opioids.i